1. Field of the Invention
The invention relates to a power amplifier stage producing a rectangular signal across a load by means of a "bridge" type amplifier comprising two pairs of bipolar transistors of the same conductivity type, a first pair of transistors having their respective emitters connected to the terminals of the load and to the respective collectors of the transistors of a second pair of transistors, a plurality of switching devices alternately supplying an amplitude control voltage to the respective bases of the transistors of the first pair and synchronously changing the base bias of the transistors of the second pair.
2. Discussion of the Related Art
A power stage of the type defined in the opening paragraph is known from the document EP-A-0,587,945. According to said document a rectangular-type signal is generated across a load by means of eight switches, of which four switches are actuated in phase with one another and four are actuated in phase opposition thereto. The amplitude of the signal obtained across the load is regulated by switching a capacitance so as to connect it alternately, by means of two change-over switches, across the load and to the input of a feedback amplifier for comparison with a reference voltage.
The known amplifier stage has been designed to drive a load whose impedance is resistive and, as the case may be, inductive and so as to avoid instability owing to the negative feedback of the amplifier and the parasitic capacitances of the circuit. It is not very suitable for driving a load whose impedance is mainly capacitive, as in the case of a piezoelectric transducer for sound-signal generation. Since no direct current flows through a load of this type the voltage signal obtained by means of the known amplifier stage will be highly distorted with a substantial loss of harmonics of the signal. However, to improve the acoustic efficiency of a transducer of this type it is required to generate upper harmonics, particularly in the case of ringing signals as, for example, in a telephone set.
Moreover, the amplifier stage known from the document cited proves to be comparatively complicated, particularly with respect to the large number of switches used for generating the rectangular pulse signal across the load.